In a typical cellular network, also referred to as a communication system or wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside operator's network to which the operator's RAN and CN provide access. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, or laptops with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wired or wirelessly in the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The communications network covers a geographical area which is divided into cell areas. Each cell area is served by a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. A “cell” is characterized in e.g. Long Term Evolution (LTE) by a “cell-ID”, which affects several cell-specific algorithms and procedures.
The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on cell size.
The base station communicates, over radio carriers or channels, with one or more user equipment(s) using a radio access technology, such as e.g. LTE, LTE Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communications (GSM), or any other Third Generation Partnership Project (3GPP) radio access technology. LTE is used as an example in the following description.
When the base station receives, at its antenna(s), signals from a plurality of user equipments, it may use different reception techniques for demodulation. Two different reception techniques for demodulating the symbols from multiple user equipments in each cell are Successive Interference Cancellation (SIC) and Interference Rejection Combining (IRC). Both of these reception techniques require a baseband receiver, at the base station, to estimate the channel between each user equipment and each base station antenna. Baseband refers to signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency. Baseband may also be used as a noun for a band of frequencies starting close to zero. The quality of the channel estimates greatly influences the performance of both SIC and IRC.
The base station may comprise multiple antennas, and the base station may receive signals from a user terminal at the multiple antennas. To receive a signal from a specific user equipment, the base station determines the set of base station antennas that will be used to receive the signal transmitted from the user equipment. The signals received by this set of antennas are sent to an “uplink receiver” that demodulates the signal transmitted by the user equipment. Note that the same set of antennas could be used for the reception of multiple user equipments. The uplink receiver typically estimates the uplink channels between each user equipment and base station antenna using reference signals that are transmitted from each user equipment on the uplink. When the base station estimates the uplink channel from a particular user equipment, the reference signals from other user equipments in the network act as interference and degrade the accuracy of the channel estimation. Therefore, it is generally desirable that the reference signals from all the user equipments are mutually orthogonal. In an LTE system, given one reference signal spanning consecutive subcarriers, a second orthogonal reference signal spanning the same subcarriers may be generated by adding a linear phase rotation to the same base reference signal. By using different phase rotations for different user equipments, a large number of mutually orthogonal reference signals spanning the same subcarriers may be generated.
The communication between a base station and a user equipment may be structured in different ways, depending on the technology which is used. For example, in LTE, the communication is structured in frames and subframes. One type of LTE frame, i.e. Time Division Duplex (TDD) mode, has an overall length of 10 ms. The frame is divided into 20 individual slots. A subframe comprises two slots, i.e. there are ten subfames within a frame. Another type of an LTE frame, i.e. Frequency Division Duplex (FDD) mode, comprises two half frames, each having an overall length of 5 ms. And each half frame is split into five subframes, each 1 ms long.
An LTE communications network is designed to support user equipments from different releases, i.e., Rel-8/9/10/11, in a backward compatible way. One of the LTE network design objective is to enable co-scheduling of such user equipments in time, frequency and space, i.e. Multi-User Multiple Input Multiple Output (MU-MIMO), dimensions with as few scheduling constraints as possible.
Furthermore, the LTE standard should be able to support various and flexible deployments. Some examples of expected deployments for modern LTE networks, i.e. Rel-11 and beyond, comprise, e.g.,                Macro-deployments, where large cells are typically divided into independent sectors.        Hetrogenous Networks (HetNet)-deployments, where pico-cells are deployed within the coverage of macro-cell in order, e.g., to improve coverage for high data rate user equipments.        Hotspot scenarios where an access point serves a small area with high throughput need.        
In addition, LTE networks are designed with the aim of enabling optional Coordinated Multipoint Processing (CoMP) techniques, where different sectors and/or cells operate in a coordinated way in terms of, e.g., scheduling and/or processing. An example is uplink CoMP where the signal originating from a single user equipment is typically received at multiple reception points and jointly processed in order to improve the link quality. Uplink joint processing, also referred to as uplink CoMP, allows transformation of what is regarded as inter-cell interference in a traditional deployment into a useful signal. Therefore, LTE networks taking advantage of uplink CoMP may be deployed with smaller cell size compared to traditional deployments in order to fully take advantage of the CoMP gains.
The uplink of LTE is designed assuming coherent processing, i.e., the receiver is able to estimate the radio channel from a transmitting user equipment and to take advantage of such information in a detection phase, i.e. in demodulation of a received signal. Therefore, each transmitting user sends a Reference Signal (RS) associated to each uplink data channel, i.e. Physical Uplink Shared Channel (PUSCH). The reference signal may also be called pilot signal and are inserted in the transmitted signal. The reference signals are sent fairly often as the channel conditions change due to fast fading and other changes.
Each reference signal is characterized by a group-index and a sequence-index. The reference signal is derived from a base sequence. Cyclic shift may be used for deriving the reference signal from the base sequence. In other words, multiple reference signal sequences are defined from each base sequence.
Base sequences are cell-specific in Rel-8/9/10 and they are a function of the cell-ID. Different base sequences are semi-orthogonal. The reference signal for a given user equipment is only transmitted on the same bandwidth of PUSCH, and the base sequence is correspondingly generated so that the reference signal is a function of the PUSCH bandwidth. For each subframe, two reference signals are transmitted, one per slot.
There are two types of uplink reference signals: a demodulation reference signal (DMRS) and a Sounding Reference Signal (SRS). The demodulation reference signal is used for channel estimation for data demodulation, and the sounding reference signal is used for user scheduling.
Reference signals from different user equipments within the same cell potentially interfere with each other and, assuming synchronized networks, even with reference signals originated by user equipments in neighboring cells. In order to limit the level of interference between reference signals different techniques have been introduced in different LTE releases in order to allow orthogonal or semi-orthogonal reference signals. The design principle of LTE assumes orthogonal reference signals within each cell and semi-orthogonal reference signals among different cells, even though orthogonal reference signals may be achieved for aggregates of cells by so called “sequence planning”.
Orthogonal reference signals may be achieved by use of Cyclic Shift (CS) in Rel-8/9 or by CS in conjunction with Orthogonal Cover Codes (OCC) in Rel-10. It is assumed that CS and OCC may also be supported by Rel-11 user equipments.
Cyclic shift is a method to achieve orthogonality based on cyclic time shifts, under certain propagation conditions, among reference signals generated from the same base sequence. Only eight different cyclic shift values may be indexed in Rel-8/9/10, even though in practice less than eight orthogonal reference signals may be achieved depending on channel propagation properties. Even though cyclic shift is effective in multiplexing reference signals assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering user equipments employ another base sequence.
OCC is a multiplexing technique based on orthogonal time domain codes, operating on the two reference signals provided for each uplink subframe. The OCC code [1-1] is able to suppress an interfering reference signal as long as its contribution after the base station matched filter is identical on both reference signals of the same subframe. Similarly, the OCC code [1 1] is able to suppress an interfering reference signal as long as its contribution after the base station matched filter has an opposite sign respectively on the two reference signals of the same subframe. The matched filter will be described in more detail below.
While base sequences are assigned in a semi-static fashion, CS and OCC are user equipment specific and dynamically assigned as part of the scheduling grant for each uplink PUSCH transmission.
Even though joint processing techniques may be applied for PUSCH, channel estimates based on reference signals are typically performed in an independent fashion at each reception point, even in case of uplink CoMP. Therefore, it is crucial to keep the interference level at an acceptably low level, especially for the reference signals.
In order to minimize the impact of burst interference peaks on reference signals, interference randomization techniques have been introduced in LTE. In particular:                Cyclic shift randomization is always enabled and generates random cell-specific cyclic shift offsets per slot. The pseudo-random CS pattern is a function of the base sequence index and the cell-ID and is thus cell-specific.        Sequence hopping and Group Hopping (SGH) are base sequence index randomization techniques which operate on a slot level with a cell-specific pattern, which is a function of the cell-ID and sequence index.                    For Rel-8/9 user equipments, SGH may be enabled/disabled on a cell-basis.            For Rel-10 user equipments, SGH may be enabled in a user equipment specific fashion.                        
In the uplink for LTE Rel-10 multi-antenna techniques which may significantly increase the data rates and reliability of a communication system is introduced. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This result in a MIMO communication channel and such systems and/or related techniques are referred to as MIMO.
LTE Rel.10 supports a spatial multiplexing mode, i.e. Single User-MIMO (SU-MIMO), in the communication from a single user equipment to the base station. SU-MIMO is aimed for high data rates in favorable channel conditions. SU-MIMO comprises the simultaneous transmission of multiple data streams on the same bandwidth, where each data stream is usually termed as a layer. Multi-antenna techniques such as linear precoding are employed at the transmitter in order to differentiate the layers in the spatial domain and allow the recovering of the transmitted data at the receiver side.
Another MIMO technique supported by LTE Rel.10 is MU-MIMO, where multiple UEs belonging to the same cell are completely or partly co-scheduled on the same bandwidth and time slots. Each UE in the MU-MIMO configuration may possibly transmit multiple layers, thus operating in SU-MIMO mode.
In case of SU-MIMO it is necessary to allow the receiver to estimate the equivalent channel associated to each transmitted layer of each user equipment in order to allow detection of all the data streams. In case of CoMP, such requirement applies also to user equipments belonging to other cells but comprised in the joint processing cluster. Therefore, each user equipment need to transmit a unique reference signal at least for each transmitted layer. The base station receiver is aware of which reference signal is associated to each layer and performs estimation of the associated channel by performing a channel estimation algorithm. The estimated channel is then employed by the receiver in the detection process.
In case of MU-MIMO, user equipments may be scheduled on fully or partially overlapping bandwidths. Some typical application cases are exemplified in the following:                MU-MIMO within a cell, fully overlapping bandwidth: in this case the reference signals of the different user equipments may be multiplexed by means of CS and/or OCC. Furthermore, SGH may be enabled without affecting orthogonality.        MU-MIMO within a cell, partly overlapping bandwidth: in this case the reference signals of the different user equipments may be multiplexed by means of OCC only and SGH cannot be enabled for any of the user equipments.        MU-MIMO of user equipments belonging to different cells, e.g., in a CoMP application,: in this case the user equipments are typically assigned different base sequences and orthogonality may not be achieved, due to the different CS hopping patterns.        
The deployments described above and the extensive use of uplink CoMP require scheduling flexibility and improved channel estimation quality, even for geographically far away user equipments belonging to another cell. Assuming, e.g., a HetNet deployment, the small cell radius of the picocell and the geographic location within the macrocell coverage implies the presence of potentially strong interference between user equipments belonging to such cells. Densifications of the cells, increasing number of receive antennas and optional CoMP processing, on the other hand, emphasizes the need for flexible MU-MIMO scheduling. In the scenarios described above, disabling SGH will enhance the risk of inter-cell interference peaks.